The conflict over Ukraine's future escalated on Thursday into the bloodiest day of violence since protests began, as the opposition routed thousands of riot police to regain control of central Kiev amid signs that the power base of embattled president Viktor Yanukovych was under threat.

Dozens died and hundreds were injured in a day of dramatic violence that turned into a seesaw contest and saw thousands of riot police scuttling from territory they seized on Tuesday. The day ended with thousands of Kiev residents patiently building city centre barricades in the cold and the dark.

Police deployed snipers and used live ammunition in a menacing escalation of the violence.

Guardian reporters saw 21 corpses on Independence Square, the crucible of the mass rebellion against Yanukovych, and in a nearby hotel converted into a makeshift field hospital. But the full death toll was impossible to verify: Oleh Musiy, head doctor for the opposition movement, said 70 protesters died on Thursday, bringing the death toll in 72 hours to about 100. The health ministry said 67 people had been killed and 562 wounded since Tuesday. The interior ministry said three police were killed on Thursday .

People gather around protesters who were killed in clashes with police in Kiev. Dozens died as the violence in Ukraine escalated. Photograph: Laszlo Beliczay/EPA

As Moscow encouraged Yanukovych to crack down harder on the unrest and threatened to withhold crucial financial aid unless he did, and the European Union announced limited sanctions on individual Ukrainian officials, three EU foreign ministers spent almost five hours with the president, desperately seeking a way back from the brink through a compromise between his increasingly hardline regime and opposition leaders.

That would include the key opposition demand for early presidential elections, something Yanukovych has shown no sign of conceding since the trouble erupted in November.

They spoke of "possible signs of progress" after seeing the president. But Thursday's escalation occurred within hours of Yanukovych calling a truce in the dangerous spiral of violence that is also spreading beyond Kiev, splitting the country east to west, and raising fears of Ukrainian meltdown.

While the authorities blocked trains coming to Kiev from the anti-Yanukovych west, protesters in the east lay down on railway tracks to prevent the government transporting military reinforcements to the capital. Crimea, ardently pro-Russian if part of Ukraine, issued threats of secession should the country go into freefall. Reports from the west spoke of protesters ransacking military and police headquarters and seizing weapons, while the security services were said to be shredding documents in scenes that recalled the anti-communist revolutions of 1989 in Romania or East Germany.

But the authorities were willing to hit back hard by bringing in the army. "Military servants of the armed forces might be used in anti-terrorist operations on the territory of Ukraine," a defence ministry statement warned.

The security service announced national "anti-terror" operations, revealing that the authorities were struggling to maintain their grip. "In many regions of the country, municipal buildings, offices of the interior ministry, state security and the prosecutor general, army units and arms depots are being seized," said Oleksandr Yakimenko, the head of the state security service.

The mayor of Kiev, Volodymyr Makeyenko, a Yanukovych appointment, announced he was quitting the president's political party. "The events happening in the Ukrainian capital are a tragedy. I have decided to resign from the Party of the Regions." Ten Yanukovych MPs broke ranks and demanded mediation in the spiralling conflict by the EU and the US.

In Kiev, Pavel, 25, a masseur, who took part in Thursday's battle equipped with body armour, balaclava, sledgehammer, walkie-talkie, flares, and a knife, said: "We live for today. We have no idea what will happen tomorrow. We need a better life for Ukraine. For our children. This is just not normal."

The battle erupted as dawn broke on Thursday when radical street fighters among the protesters attacked and broke through the police lines established on Independence Square on Tuesday. A firefight left at least 10 dead, including nine opposition fighters. The militants then surged out of the square up the hill to the south where police snipers could be seen picking out targets as the city centre turned into a warzone.

The deafening noise included the clear sound of automatic weapons, as well as smoke and percussion grenades raining down on an avenue leading away from the square towards the parliament and the central bank. Demonstrators also reoccupied government buildings evacuated earlier in the week. Busloads of common riot police swiftly deserted the scene of the battle to be replaced by the special units of the Berkut security service. They, too, retreated very quickly, allowing the protesters to advance.

Dead and wounded were hauled away on their backs, on wooden planks, on makeshift metal shields and in blankets. Corpses lay temporarily abandoned on the streets. Police vehicles were set ablaze and then hacked to pieces.

Protesters ducked behind trees and ran for cover as police opened automatic gunfire. But by mid-morning the city centre was firmly in the hands of the opposition. The police seizure of the part of the square which cost 28 lives on Tuesday was finished, however temporarily. Some 60 riot police surrendered or were "taken prisoner" when the protesters stormed the police lines.

The security forces pulled back around two kilometres to form new lines. For the rest of the day the city's residents turned out to bring the hard core protesters food and drink and to help in erecting massive barricades.

Credit: Guardian graphics

At night, human chains of men and women, young and old, lined up to pass along tyres and bricks, rubble and debris that were wheelbarrowed in to build huge blocking points around the city centre in an attempt to keep the security forces at bay.

"I was watching it on TV and decided to come and help," said Yuri Kugno, 48, who works in a curtain design firm. "I'm going to build a barricade now."

"What is happening right now in Ukraine is criminal and anti-human," said Dr Olga Bogomolets, a professor of medicine. "All the people killed here had no guns or arms."

Her colleague, Natalya Hot, a gynaecologist and a hospital deputy director, , said of the ordeal she witnessed: "I still have some protective reaction in my brain as a doctor. But it was a horror. I could not imagine I would see what I'm seeing today in my life in my country."

"I accompanied them to make sure they were not beaten but forgiven," said Nikolai Himaylo, a Ukrainian Orthodox priest who administered the last rites to some of the dead. "I'm a witness to what has become a criminal state," he said. "Yanukovych cannot be forgiven. These boys are dying for freedom."

Dependent on Russian money and gas supplies since he spurned trade and political pacts with Europe in November, the spark for the crisis, Yanukovych was told by Moscow to maintain a hard line and warned that the financial aid could be turned off if he did not.

Dmitry Medvedev, the Russian prime minister, said that Yanukovych would have to restore order to qualify for the Russian help and that if he did not the opposition forces would use him as "a doormat".